Soil Moisture Calculator
Analyze soil moisture characteristics including available water capacity, current plant-available water, moisture deficit, and irrigation need based on soil type and root zone depth.
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The Soil Moisture Calculator helps farmers and irrigation managers assess soil water status and make informed irrigation decisions. By selecting your soil type and entering current moisture content and root zone depth, you can calculate available water capacity, deficit, and the need for irrigation.
Understanding Soil Moisture Management
Soil moisture management is critical for optimal crop growth and water conservation. The three key thresholds are saturation, field capacity, and permanent wilting point. Plant-available water is the moisture held between field capacity and wilting point. Water above field capacity drains away and is unavailable to plants, creating potential for nutrient leaching.
Available Water Capacity (AWC) varies by soil texture. Sandy soils have low AWC (0.5-1.0 in/ft) but allow rapid infiltration. Clay soils have higher AWC (1.5-2.5 in/ft) but slower infiltration. Loam soils provide the best balance with moderate AWC and good drainage. Understanding your soil's AWC helps determine irrigation frequency and application amounts.
Management Allowable Depletion (MAD) defines how much of the available water can be used before crop stress occurs. For most field crops, MAD of 50% is common meaning irrigation should occur when half the available water has been used. For high-value or shallow-rooted crops, lower MAD values (30-40%) prevent stress and maintain quality.
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Frequently Asked Questions
What is field capacity?
Field capacity is the amount of water held in soil after excess water has drained away, typically measured 24-48 hours after a saturating rain. It represents the upper limit of plant-available water. At field capacity, water is held in soil pores against gravity but is available for plant uptake.
What is permanent wilting point?
Permanent wilting point (PWP) is the soil moisture level at which plants permanently wilt and cannot recover even when water is added. At PWP, water is held so tightly by soil particles that plant roots cannot extract it. It represents the lower limit of plant-available water.
How is available water capacity (AWC) calculated?
Available water capacity is the difference between field capacity and permanent wilting point: AWC = FC - PWP. This represents the total amount of water a soil can store for plant use. Sandy soils have low AWC (6-8%), while clay and loam soils have higher AWC (15-20%).
What is management allowable depletion (MAD)?
MAD is the fraction of available water that can be depleted before crop stress occurs. Typical MAD values are 0.40-0.50 for most crops but can be as low as 0.30 for sensitive crops (lettuce, strawberries) and as high as 0.65 for deep-rooted crops (alfalfa, trees).
When should I irrigate based on soil moisture?
Irrigation should be scheduled when current soil moisture in the root zone approaches the management allowable depletion limit. This threshold is calculated as: Irrigation Trigger = FC - (AWC � MAD). When moisture drops below this, irrigate to refill the root zone back to field capacity.